Epitaxy, epitaxial growth, and epitaxial deposition refer to growth or deposition of a crystalline layer on a crystalline substrate. The crystalline layer is referred to as an epitaxial layer. The crystalline substrate acts as a template and determines the orientation and lattice spacing of the crystalline layer. The crystalline layer can be, in some examples, lattice matched or lattice coincident. A lattice matched crystalline layer can have the same or a very similar lattice spacing as the top surface of the crystalline substrate. A lattice coincident crystalline layer can have a lattice spacing that is an integer multiple of the lattice spacing of the crystalline substrate. The quality of the epitaxy is based in part on the degree of crystallinity of the crystalline layer. Practically, a high quality epitaxial layer will be a single crystal with minimal defects and few or no grain boundaries. Traditionally, metal contact layers are applied to an epitaxial structure at some point in the upstream processing. With today's complex epitaxial structures often incorporating more than one device functionality, this can require extensive etching and deposition of metals on wafers with a large amount of topography.
Interactions between metals and semiconductors are often critical to device operation. One example of such an interaction between a metal and a semiconductor occurs in a thin film resonator such as an RF filter where the overall acoustic performance is defined by the product of the acoustic impedance of the electrode and the acoustic impedance of the piezoelectric material. In fact, to access high resonant frequencies it is essential to make both the electrode and the piezoelectric material quite thin. This is summarized in FIG. 17, which shows resonant frequencies as a function of AlN thickness for different thickness metal electrodes (from S. Tanifuji et al, Proceedings 2009 IEEE International Ultrasonic Symposium, p. 2170, the entirety of which is incorporated by reference). Here, crystal quality is also important because without it resistivity would increase as thickness decrease due to an increasing effect of defects and grain boundaries in polycrystalline metal layers.
Growth of InP has also been attempted on metal over silicon engineered substrates, as described in Zheng et al, Journal of Applied Physics, vol. 111 p. 123112 (2012), the entirety of which is incorporated by reference. However, Zheng describes films that are polycrystalline, not epitaxial.
Epitaxial growth of metals on yttria stabilized zirconia (YSZ) is described in Gsell at all, Journal of Crystal Growth, vol. 311, p. 3731 (2009), the entirety of which is incorporated by reference. Gsell describes separating the metal from the underlying silicon substrate by using YSZ as this prevents the unwanted siliciding of any epitaxial metal. YSZ is a sputtered material (or deposited with pulsed laser deposition) using zirconia and yttria targets. It is not a single crystal material, has grain boundaries, and can be of mixed crystallinity (cubic and tetragonal). Thus, it is a suboptimal template for epitaxial growth of metals. In addition, control of the YSZ/silicon interface is technically challenging.
Accordingly, epitaxially growing metal of good crystal quality over semiconductor materials has proven to be difficult.